A Quantum Theory That Peels Away the Mystery of Measurement | Hacker News Reader

A Quantum Theory That Peels Away the Mystery of Measurement

nautil.us

55 pointsdnetesn26 comments

There was a nice thread about these discoveries on physics stack exchange https://physics.stackexchange.com/questions/484675/does-the-...

Very interesting. Funnily enough, Wikipedia doesn't seem to know anything about quantum trajectory theory. It's always amusing to find something that isn't on Wikipedia.

It would appear to be very similar to the Bohmian interpretation. https://en.m.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theo...

There have been some nice papers on this. See "Experimental nonlocal and surreal Bohmian trajectories" https://advances.sciencemag.org/content/2/2/e1501466.abstrac.... They use weak measurements to make inferences about particle trajectories (ala QTT).

Maybe the authors feel that they are working from a different theoretical foundation. The foundational argument, that quantum systems have specific but unobservable states, is the same in QTT and DeBroglie/Bohm models.

Searching the internet, it seems that it existed for decades (1993, H. J. Carmichael), it simply wasn't popular:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.70...

and the 1999 paper:

https://link.springer.com/chapter/10.1007/BFb0105336

Ah, the theory of looking from results to constituent measurements, working backwards.

The article notes: "Achieving this degree of control and information capture is very challenging."

This is an extreme understatement: for anything but the simplest quantum systems in the simplest, most tightly controlled configurations, it's practically impossible. That's not to say that the QTT experiments described aren't impressive achievements; they are. But they should not be oversold either.

I think it's more than that. If I understand correctly (and I may well not), this kind of thing isn't possible within classical QM, even in principle. The fact that we can do it at all, even in perfect circumstances, is enough.

> classical QM

There is no such thing as "classical QM". If you mean pre-quantum classical physics, this kind of "information capture" certainly is possible in classical physics, regardless of how complicated the system is. It's only in quantum physics that it becomes an issue.

Yes, I know that QM is not "classical". By "classical QM", I meant regular, Schrodinger-equation QM, not "quantum trajectory theory" QM.

> By "classical QM", I meant regular, Schrodinger-equation QM, not "quantum trajectory theory" QM.

"Regular" QM is not limited to the Schrodinger Equation. You also have to use the collapse postulate and the Born Rule where appropriate, i.e., whenever measurements are made. QTT is basically just a reformulation that makes it easier to see how to make use of the information revealed by measurements.

The "Schrodinger" viewpoint referred to in the article is not the Schrodinger Equation, but Schrodinger's personal take on QM.

Are you saying that the Schrodinger Equation, the collapse postulate, and the Born Rule are mathematically equivalent to QTT?

> Are you saying that the Schrodinger Equation, the collapse postulate, and the Born Rule are mathematically equivalent to QTT?

Yes.

wait, but doesn't typical qm operate off of integrating over every possible quantum trajectory, including relativity-prohibited ones where particles take superluminal velocities? so if qtt is valid and otherwise compatible with vanilla qm, how does it address that discrepancy?

You're confusing quantum trajectories for classical ones. The latter are what we use in path-integral formulations, and constitute a single path through spacetime. No such path exists for quantum states, since that would be a local hidden-variable theory (which has been experimentally ruled-out).

Trajectories of quantum states are well described here: https://arxiv.org/abs/quant-ph/0302080

ah right, thanks. been a few years since I did any qm work last

The difference is the rate of the measurements and the data collected. Usually you would measure the results (average of sums). This is more like a Fourier transformation, where each movement is measured and the entire state is measured. Sum of averages.

This is just reversible computation, isn’t it?

This is very similar to my algorithm I have been working on. It's an connection of past and future states. It provides a translational symmetry between the events. That symmetry creates a unique pattern of the connected events. It's principal is on the banach Tarski paradox and specifically the axiom of choice. That coupled with unique constraints solves the information entropy issue. Essentially using 4/pi. ++,+-,--,-+.

If you think I'm crazy.

Phys.Org: A connection between quantum correlations and spacetime geometry. https://phys.org/news/2019-07-quantum-spacetime-geometry.htm...

I don't have time to waste wading through what is inevitably going to be a watered down and screwed up lay person's interpretation of some paper they don't understand that got some hype. I scanned the article but didn't see any scholarly citations. What's the real research behind this?

You have to jump through the link to the original article which then has a link to relevant papers, eg: https://science.sciencemag.org/content/339/6116/178

I directly found a completely wrong description of the Heisenberg uncertainty relation and stopped reading after that. But yes, the underlying research probably has some interesting stuff!

You probably mean:

"A quantum measurement influences the system being observed: The act of observation injects a kind of random noise into the system. This is ultimately the source of Heisenberg’s famous uncertainty principle."

I'd agree that's an inaccurate description of the uncertainty principle, but then the following sentences are:

"The uncertainty in a measurement is not, as Heisenberg initially thought, an effect of clumsy intervention in a delicate quantum system—a photon striking a particle and pushing it off course, say. Rather, it’s an unavoidable outcome of the intrinsically randomizing effect of observation itself."

I'm not sure I could explain it better at this level...

Well, it has to be reminded that the uncertainty principle and the randomness of the outcome of a measurement are two different things. The former involves two observables which cannot be measured simultaneously with an arbitrary precision whereas the latter is the result of an observable having more than one eigenvalue.

I have not thought this through, but it seems to me that if all observables commuted there would be no randomness and if there was no randomness all observables would commute. Does anyone see an obvious flaw on this?

I would rather these kinds of articles just explain the uncertainty principle in a wave mechanics context, without any quantum theory to obfuscate.

Or else just go "What if AB != BA?" and actually somewhat give people an entrance point to what is actually happening.

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